(1) Field of the Invention
This invention relates generally to electrical resistance measurement and relates more particularly to a very precise measurement of the resistance of small resistors using a semiconductor circuit.
(2) Description of the Prior Art
Monitoring the resistance regularly of e.g. external sensors or of safety devices as squibs for airbags or other devices for automotive or aviation applications is an important task. This measurement must be reliable, precise and fast and it may not impede the normal application of the sensor, or resistor whatever. This measurement has often to be performed almost constantly in short time intervals.
A challenge to the designers of electronic circuits is to find an efficient solution to measure the resistance of such devices periodically. There are various patents available dealing amongst other purposes with the measurement of resistances.
U.S. Patent (U.S. Pat. No. 6,133,749 to Hansen et al.) describes a programmable variable impedance output driver circuit using analog biases to match driver output impedance to load input impedance. A current mirror is used to obtain a measurement of an external resistance value for matching the impedance of a driven load. The mirrored current generates the voltage “NBIAS” when passed through the resistively connected NFET. Similarly, the current is again mirrored and passed through a resistively connected PFET resulting in the voltage “PBIAS”. The analog bias voltages, NBIAS and PBIAS are used to vary the impedance of complementary FETs in an impedance matched driver for a high degree of dl/dt control. The driver provides a high degree of flexibility because its turn-on and turn-off characteristics do not depend on a combination of digital control signals connected directly to the driving FETs as in the prior art. Instead, the PBIAS and NBIAS signals provide analog controls which may be applied to single transistors whose impedance changes as PBIAS and NBIAS increase or decrease.
U.S. Patent (U.S. Pat. No. 6,498,494 to Belau et al.) discloses how the resistance value and the leakage current of a load can be measured simultaneously. A load current is fed, mirrored by electrical power supplies into a resistance measuring circuit and a leakage current measuring circuit. This permits rapid, precise measurement of a plurality of loads by a single measuring circuit connected via one multiplexer. The firing transistors can be distributed among different ASICs or provided jointly for a plurality of firing caps.